Electrolytic potassium hydroxide descaling



United States Patent Oificc 3,254,011 Patented May 31, 1966 3,254,011 ELECTROLYTIC POTASSIUM HYDROXIDE I DESCALING Donald R. Zaremski, Cheswick, Pa., assignor to Allegheny Ludlum Steel Corporation, Brackenridge, Pa., a

corporation of Pennsylvania N Drawing. Filed Sept. 20, 1963, Ser. No. 310,504

8 Claims. (Cl. 204-1405) This application is a continuation-in-part of application Serial No. 42,964 dated July 15, 1960 entitled: Caustic Bath, now Patent No. 3,121,026.

This invention relates to the removal of oxide scale from metals, and more particularly to an improved method for loosening and removing oxide scale from stainless steel strip.

During the processing of stainless steel from an ingot to a finished strip product, the in-process material is frequently heated in oxidizing atmospheres to temperatures at which oxide scale readily forms on the surface. This oxide scale is normally removed by conventional pickling baths, the steel strip being fed through the baths at a slow enough rate to efiect removal of the oxide scale. However, when the scale-removing baths are used in conjunction with continuous annealing processes or the like, the scale-removing bath must be of suflicient strength to remove the scale as rapidly as the strip emerges from the furnace. In light gauge strip this speed may be rather rapid and since the speed of the strip is a critical factor in continuous annealing, this speed cannot be changed to accommodate the scale removal equipment. Because of this speed limitation and the tenacious nature of the scale of many types of stainless steel, special salt baths are re quired to loosen the scale and aid in its removal. These salt baths conventionally take the form of fused alkali metal hydroxides operating at temperatures from 800 F. to 1000 F. which tend to loosen the scale after which the strip is passed successively first into a bath of dilute sulfuric acid and then into a bath of mixed hydrofluoric and nitric acids. Although the high temperature salt baths are effective to loosen oxide scale, they tend to chemically attack the strip; they also have the additional disadvantages ofattacking the various components of equipment required to retain the salt and guide the strip through the bath, such as tank linings, rolls, etc., which, in turn, can cause Warping and distortion of lighter gauge strip.

In the parent application referred to above, an aqueous solution of potassium hydroxide operating at a relatively low temperature of about 400 F. is disclosed as having excellent properties in loosening and removing scale on titanium alloys and on stainless steel when used in place of the high temperature salt baths. This aqueous potassium hydroxide bath works excellently on titanium alloys and on some types of stainless steel; however, on certain types of stainless steel which form a very tenacious scale such as AISI Types 309, 310, 316 and 347, even this salt bath has proved to be too slow and inefiicient for commercial application.

It is, accordingly, a principal object of this invention to provide an improved process for loosening and removing oxide scale from metal.

A more specific object of this invention is the provision of an improved salt bath-electrolytic treatment for loosening and removing scale from stainless steel.

Yet another object of this invention is to provide an improved process for continuously loosening and remov- I removal characteristics.

ingoxide scale from strip material, the process being adapted to be incorporated at the exit end of a continuous annealing line.

Still a further object of this invention is the provision of a relatively low temperature aqueous salt bath-electrolytic the provision of an improved descaling bath for stainless steel.

These and other objects will become apparent from the following description taken in conjunction with the appended claims.

According to this invention, an aqueous solution of potassium hydroxide or an aqueous solution of potassium hydroxide and potassium nitrate is used as. a descaling bath and electric current is passed through the bath and the stainless steel or other metal being descaled to eifect descaling. (Potassium nitrate, if present, tends to increase the electropolishing capability of the bath as will be discussed presently, and has little effect on the scale In any event, the term potassium hydroxide bath as used herein is intended to include baths having potassium nitrate as well as baths free of potassium nitrate.) It has been found that the use of electric current in conjunction with an aqueous potassium hydroxide bath substantially improves descaling results over the results from similar baths wherein no electric current is used with the scale from even the most difiicult grades being removed quickly and completely. This improved result is readily apparent from a comparison of Tables I and II below, Table I showing the descaling results of an aqueous potassium hydroxide bath wherein no electric current was used and Table H showing the results of a similar aqueous potassium hydroxide bath wherein various current densities were passed through the bath and the steel. These Tables show results of controlled laboratory experiments for removing scale from small size samples of various types of stainless steel with various types of oxide scale.

TABLE I.NON-ELECIROLYTIO AQUEOUS KOH DE- SCALING Alloy (AISI Type Scale KOH Treatment 1 Descaling Grade) Results Finish Anneal 1 and 2 miu. Compilgte at 2 m -do land 2mi.n Complete at 1 min. do Do. do Complete. do Incomplete. do Do. Intermediate A eal Do. Finish Anneal. Do. Intermediate AnneaL. Do. Finish Anneal Do. do Do. do Complete at 2 min. 430 do 1 and 2mm Complete at 1 min. Intermediate AnneaL. 2 min Complete. 430 Hit Ro1l-Normelize 2 min Incomplete.

. nnea l Caustic treatment in an aqueous solution of KOH, 10% KN O; and 10% E20 maintained at about 425? F. followed by a 2 minute immen sion in 15 percent H2804 water solution at 150 F., and a 2minute immersion in 10 percent HNO .2 percent HF water solution at F.

TABLE II.(ELECTROLYTIC) AQUEOUS KOH DESCALING KOH Alloy Treatment 1 (AISI Type Scale Time, Anodie Descahng Results Grade) min. Current Density, amp/in.

201 Finish Anneal 1 0. 125 Complete 301..- do 1 0.125 D0. 304--. do 1 0.125 D0. 3l0 do 1.5 0. 250 Do. 310. Intermediate AnneaL. 1. 5 0. 350 D0. Finish Anneal 1. 5 0. 250 Do. Intermediate Anneal 1. 5 0. 350 Do. do 2 0. 750 Complete plus polished surface.

Finish Anneal 1. 5 0.250 Complete. do 1.5 0. 250 Do. do 1 0. 250 Do. do 1 0. 125 Do. terme 1. 5 0.350 Complete plus bright surface. do 2 1. 000 Complete plus polished surface. Hot Roll-Normalize- 2 0. 350 Incomplete.

Box Anneal. do 5 1.000 Complete.

From Table I it can be readily seen that with an aqueous potassium hydroxide bath satisfactory descaling results are obtained on stainless steel having scale that is relatively easily removed, such as AISI Types 201, 301, 302, 304 and 410, and certain types of scale on Type 430 without the use 'of electric current. However, 'on the material have scale which is'difiicult to remove, such as Types 309, 310, 316, 321 and 347 and some types of scale on Type 430, the results listed in Table I clearly show that the use of an aqueous potassium hydroxide bath without electric current will not completely remove the scale in two minutes. Because of the required use of descaling in conjunction with other processes such as continuous annealing, a time requirement of greater than tive electrodes (anodes), and thence into the second compartment in which were disposed negative electrodes (cathodes). This bi-polar type of bath was used to insure consistent results in preference to a single bath having a positive roll contacting the strip and negative grids in the bath. With the contacting roll maintaining the strip positive throughout the bath better scale removal results, but arcing at the roll may be a problem with smaller diameter rolls and heavier gauge strip, especially at higher current densities, hence, the preference for the bi-polar bath. The current densities were varied as shown in Table III, producing the results indicated therein. Two tanks A and B following the potassium hydroxide bath were filled with acid as indicated.

TABLE III.CONTINUOUS AQUEOUS KOH DESCALING OF TYPE 316 STAINLESS STEEL Acid Treatment Caustic Treatment @peed, Results and Comments .p.m. Tub A Tub B Non-Electrolytic 15% H1804 at 140 F 7% HNO;2% HF at 100 F 2 No descaling.

Do 15% H 804 at 140 F. and 0.5 do 2 D0.

amp/111. Do 5% 1128942812 130 F. and 1.0 7% HNO32% HF at 98 F 2 -Do.

amp. 1n. Do 5% H280 at 140 F. and 0.5 10% HNOa at 130 F. and 0.5 1. 3 Do. amp/in]. amp./in. Electrolytic at 0.25 amp/in. 5% 1128/94 at 95 F. and 0.25 7% EN 03-2 HF at 125 F 1. 3 Desealmg complete.

amp. 111. Do o No acid- 1.3 Do. Do 5% H2304 at 95 F 7% HNO32% HF at 125 F 1. 8 Do. Do. rio No acid- 1. 3 Do. Dodo 2 Do. Do do 3 Do. Electrolytic at 2 amp/inkdo 1. 3 Descaling complete with surface electropolish.

1 Aqueous solution of 80% KOH, 10% KNO: and 10% 1110 maintained at about 425 F.

two minutes tends to negative the commercial practicability of the process. The data in Table II shows when electric current is used in conjunction with the aqueous potassium hydroxide-potassium nitrate bath, the scale is easily removed even on most of these diflicult grades within the two-minute requirement and, hence, is applicable for commercial production.

In order to establish the commercial feasibility of this process, several strips of A181 Type 316 stainless steel (a difficult grade) having oxide coatings, were passed continuously through an aqueous solution of potassium hydroxide at various speeds with varied amounts of electric current being used and several different acid dips being given following the potassium hydroxide treatment. The electric current was applied by means of a bi-polar system in which the tank of aqueous potassium hydroxide was divided into two compartments. The strip passed descale even difiicult alloys at the required rapid line speeds. Also, from an examination of Tables II and III, it can be seen that the conventional dual acid treatment following the potassium hydroxide treatment can be eliminated and a single treatment in aqueous 5% sulfuric acid is sufficient to remove the stain from the surface of the strip. Further, there is no need for passing any current through the sulfuric acid bath as has been the prior art practice. Table IV below shows the descaling results of various concentrations of potassium hydroxide into the first compartment in which were disposed posiand potassium hydroxide-potassium nitrate baths on de- 6 scaling of Type 316 stainless steel and also shown for but also will be oxidized to a form which will readily sepacomparison are sodium hydroxide and sodium hydroxiderate from the surface of the strip and enter the potassium sodium nitrate baths. I hydroxide bath. But, in the two-tank set-up described,

Mariam-K011 vs. NaOH DESOALING BATH EVALUATION [Type 316 Stainless Steel P .25 amp./in. .50 amp/in. 1.0 amp/in. 2.0 amp/in. 3.0 amp/in.

er- Percent Percent cent Temp, Caustic Nitrate H2O F. Percent Bright- Percent Bright- Percent Bright- Percent Bright- Percent Brightscale ness 3 scale ness 2 scale ness 2 scale ness 2 scale ness 2 removal removal removal removal removal 400 100 22 100 24 100 100 32 100 36 20 400 100 18 100 20 100 12 100 16 100 18 400 90 6 9O 7 80 7 85 6 90 7 315 80 5 85 6 85 6 85 6 85 5 250 90 6 90 6 85 6 80 4 75 4 250 90 7 90 7 90 6 90 7 90 7 230 85 7 85 6 85 6 80 7 80 6 220 50 3 50 2 50 2 50 3 50 3 5 400 100 23 100 18 100 32 100 29 100 33 10 400 100 20 100 13 100 30 100 30 100 32 10 400 100 15 100 12 100 I 17 100 16 100 22 10 400 100 12 100 9 100 6 100 4 100 4 20 400 85 8 85 6 85 5 85 6 90 7 20 400 3 5 85 6 85 5 80 4 10 400 7 90 8 90 7 100 7 100 6 30 365 80 5 80 5 80 5 80 4 85 4 58 )240 80 5 80 5 80 5 85 5 80 5 1 3 20 400 75 3 70 3 70 3 30 350 75 4 75 4 75 5 70 4 70 3 40 325. 80 5 80 5 80 6 75 5 75 3 50 285 80 4 B5 7 80 5 75 4 75 5 90 NaOH. 5 NaNOi 5 80 NaOH l0 NaNOs. 10 70 NaOH 20 NaNO3 10 60 NaOH 20 NaNO3. 20 400 90 5 5 95 5 90 5 90 5 50 NaOH 20 NaNO 30 330 90 6 90 6 85 4 85 5 90 6' 25 NaOH--- 25 NaNO 50 245 90 7 95 6 9O 5 5 85 5 1 Each sample was given a two minute treatment in the bath indicated at the temperature and current density indicated and then rinsed in a 5% aqueous solution of sulfuric acid.

2 Surface reflectivity readings as measured by gloss meter sensing head, Model N o. 660M and recording unit, Model No. 610 manufactured by Photovolt Corporation, New York, New York.

3 Could not get NaOH into solution at operating temperature or above.

4 Could not get NaOH and N aNOa into solution at operating temperature or above.

From Table IV above, it can be seen that in order to 40 the strip must be cathodic during at least a portion of the achieve descaling in two minutes, which for comprocess. This portion when the strip is cathodic should mercial reasons is the longest permissible time, an aquebe kept to a minimum since it is not as effective for scale ous potassium hydroxide bath free of nitrate additions removal and since electropolishing will not occur when must have at least an 80% concentration of potassium the strip is cathodic. With the two compartment arrangehydroxide and when potassium nitrate is present in the 45 ment of the tank as described above, the strip is mainbath, there must be at least 60% potassium hydroxide tained cathodic in the first compartment and anod-ic in the present and the combined percentage of potassium hydroxsecond.

ide and potassium nitrate must be at least 90%. It can Although several embodiments of this invention have also be seen from Table IV that aqueous sodium hydroxbeen shown and described, various adaptations and modiide baths and sodium hydroxide-sodium nitrate baths are 50 fications may be made without departing from the scope not efiective in any concentration to remove 100% of the and appended claims.

scale in two minutes. Another benefit that can be derived I claim:

from the electrolytic descaling in the baths of this inven- 1. In a process for removing oxide scale from stainless tion is an electropolish-mg of the surface of the steel in steel theimprovement which comprises, immersing said addition to scale removal. This can be seen from Table 55 steel in an aqueous bath having at least 80% potassium IV in which gloss readings as high as 36 are produced on hydroxide and no less than 5% water, maintaining said the stainless steel. This is a substantial amount of polishbath at a temperture of less than 500 F., and passing an ing since the original surface gloss of the material before electric current through said bath and said steel.

scaling was less than about 10. It should be noted that 2. The process of claim 1 wherein the current density n ppre i l m n of l po hing occurs wh n 60 is maintained at a value of at least .125 ampere per square the potassium hydroxide content is below 70% such as i h of Steel rfa e i d i h b h,

when there is 60% potassium hydroxide and 30% P 3. The process of claim 2 wherein the steel is immersed Slum mate, and even at 70% Potassium hydroxide and in a second 'bath of dilute acid after scale removal;

20% potassium nitrate, the surface gloss is not raised to above 20 until the current density is at least 3 amps. per 6 square inch.

It has been found that there must be at least 5% water in the bath for the bath to 'be suitably electrically iconduct- In aprocess for remqvmg oxlde. scalfi from i i ing. Also, the temperature of the bath should not exceed Steel the Improvement whlch comprises lmmersmg sald about 500 F. or the water will be driven off and the bath 70 steel in an aqueous bath having at least 60% Potassium 4. The process of claim 2 wherein at least a portion of the strip in the bath is maintained anodic whereby to electropolish the strip.

will become anhydrous and unsuitable for the practice of hydroxide, and from 5% to 30% Potassium n ate and thi inv ti n, wherein the total amount of potassium hydroxide and It is preferred, in the practice of this invention, that Potassium nitrate is from 90% to 95 maintaining Said the strip be anodic or positive during at least a portion a h t a temperature of less than and p g n of the treatment so that the scale will not only be loosened 75 electric current through said bath and said steel.

7 3. 6. The process of claim 5 wherein the current density References Cited by the Examiner is maintained at a value of at least .125 ampere per square UNITED STATES PATENTS Steel surfacelmnlersed bath 2,797,193 7/1957 Eigler et a1 204 143 7. The process of claim 6 wherein the steel is immersed 3 096 261 7/1963 Mekjean in a second bath of dilute acid after scale removal. 5 3:121:026 2/1964 Beigay 8. The process of claim 6 wherein at least a portion of the strip in the bath is maintained anodic whereby to JOHN MACK Primary Exammerelectropolish the strip. R. HARDER, R. MIHALEK, Assistant Examiners. 

1. IN A PROCESS FOR REMOVING OXIDE SCALE FROM STAINLESS STEEL THE IMPROVEMENT WHICH COMPRISES, IMMERSING SAID STEEL IN AN AQUEOUS BATH HAVING AT LEAST 80% POTASSIUM HYDROXIDE AND NO LESS THAN 5% WATER, MAINTAINING SAID BATH AT A TEMPERATURE OF LESS THAN 500*F., AND PASSING AN ELECTRIC CURRENT THROUGH SAID BATH AND SAID STEEL. 